A 11 ID 2 155754 NAT'L INST. OF STAND & TECH

I m

NBS

Reference PUBLICATIONS

NBSIR 84-3004

AlllDb 5 2 fl fl 1

A

THE NBS SWITCHING RADIOMETERS

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

May 1984

QC—

100

.056

84-3004

1984

y

NBSIR 84-3004

THE NBS SWITCHING RADIOMETERS

Electromagnetic Fields Division

National Engineering Laboratory

National Bureau of Standards

U.S. Department of Commerce

Boulder, Colorado 80303

May 1984

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

C.K.S. Miller

W.C. Daywitt

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

CONTENTS

Page

1. INTRODUCTION 1 1.1 What Is Measured? 1

2. ERROR ANALYSIS OF THE SWITCHING RADIOMETER 2 2.1 The Switching Concept 2 2.2 Fundamentals of Error Analysis 4 2.3 Error Analysis of a NBS Type Switching Radiometer 6

3. THE NBS SWITCHING RADIOMETERS 9 3.1 The WR90 and WR62 Radiometers 9 3.1.1 The WR90 Comparison Radiometer 9 3.1.2 Applying the Error Analysis to the WR90 Radiometer 13

4. WR90 AND WR62 MEASUREMENT SERVICES 16

4.1 Existing Waveguide Measurement Services 16

THE NBS SWITCHING RADIOMETERS

C. K. S. Miller and W. C. Daywitt

National Bureau of Standards

Boulder, Colorado 80303

An error analysis for the Dicke radiometers used by the National Bureau of

Standards (NBS) in their WR90 and WR62 waveguide noise calibration services for sources with noise temperatures above 1000 kelvin is discussed. A list of meas- urement frequencies currently available in the WR90 and WR62 bands is presented.

Key words: error analysis; microwave; noise; radiometer.

1. INTRODUCTION

The material presented in this report was extracted from notes presented at a Noise Seminar

conducted in 1970, and represents the "state of the art" at that time.

1. 1 What Is Measured?

The measurement of noise sources is usually made by comparing a "standard" noise source with

an "unknown noise source." The comparison is made by a radiometer. There are many types of radiometers and each has its own advantages and disadvantages. For comparing microwave noise sources

that have outputs above room temperatures with standards above room temperature, a switching type

of radiometer is one of the types employed at NBS [1,2]. The "standard noise source" and "unknown

noise source" in this case is the NBS Reference Standard and the NBS Working Standard respectively

during the initial comparison, after which the noise output of the NBS Working Standard is known

and has a specified accuracy. For the next step the "standard noise source" is the NBS Working

Standard and the customer's noise source is the "unknown noise source." For convenience when

discussing comparisons the "standard noise source" and "unknown noise source" are consistently

referred to as standard and unknown respectively, and the level of comparison can be judged by

what noise source is used as the standard.

At NBS the reference noise standard is used to determine the output of three working standards. Each customer's noise source is measured using two of the three working standards. The

noise source submitted for calibration is compared to the working standard, and the comparison

relates the difference in noise power output between these two items through a difference attenuation measurement. Since the noise power is related to effective noise temperature, T ne

, of the

item by

P noise kT B ne (1)

(where k is Boltzmann's constant and B is the limiting bandwidth of the system), it is established

practice to report to the customer the value of the effective noise temperature of the output of

1
the noise source calibrated. The effective noise temperature [2], T

ne

, is proportional to the

power emerging from the output port of a noise source when it is connected to a nonreflecting

load. The noise temperature, T

n

, of the noise source (generator) is analogous to the available

power from a generator [3] and is obtained when corresponding reflection coefficients for generator and load are complex conjugates of each other. The relationship between the noise temperature and effective noise temperature is

T ne = d - ' r ' 2 )T

n

,

where r is the magnitude of the reflection coefficient of the customer's noise source (generator).

For this comparison, that is between customer's noise source and the working standard, the radiometer input port is adjusted to have a reflection coefficient magnitude less than 0.01.

The noise output of a noise source can also be reported in terms of excess noise ratio, ENR. The excess noise ratio, expressed in decibels, is calculated from the effective noise temperature

using the equation

ENR = 10 log

1Q

where T

q is 290 K, which is an arbitrary standard temperature [4],

This measurement process can only accommodate continuously operated noise sources and has not

been adapted to measuring pulsed gas-discharge noise sources for three reasons: 1) pulsed noise sources are very unstable even in the pulsed mode of operation (this problem is further compounded

by the feature that their noise output varies widely depending upon where and for how long in the

pulse the noise is sampled); 2) there is no common or universally accepted pulsing rate and gating characteristic used by the manufacturers of automatic noise figure meters; and 3) pulsed

noise sources generally have shorter useful lives.

Using calibrated noise sources, noise figures, noise factors, operating noise temperatures,

and effective noise temperatures of devices, components, and systems can be measured by either the

Y-factor or other similar techniques [5]. An automatic noise figure meter could be devised that

would be quite accurate if it employed a switchable ferrite to simulate a pulse of noise by switching the RF output of a noise source operated in the continuous mode. It should be noted that

automatic noise figure meters are at present only relative measuring instruments.

2. ERROR ANALYSIS OF THE SWITCHING RADIOMETER

2.1 The Switching Concept

A switching radiometer is basically a half power radiometer and is often called a Dicke radiometer, since in 1946 Dicke [6] suggested using a switch in the front end of a radiometer (fig.

la). The name half power arises because only half the power available at the input is available

for use by the receiver. This situation results due to the switching process connecting the re- ceiver to the source being observed for half the time and to some reference source for the other

half of the time. Dicke constructed a wheel made of an absorbing material and shaped in such a way that, when rotated by a motor in a slotted section of the waveguide, it produced a nearly

square wave modulation with close to equal times in and out of the waveguide. If it is assumed
UJ<

3

Figure

la.
that the wheel and antenna are both nonreflecting, then the effect of the wheel is one of disconnecting the antenna and connecting an equivalent resistance (the absorbing material of the wheel)

to the receiver. Note that if the radiation from the wheel and the antenna is the same no change

will be noticed on the output meter, while if there is a difference a modulated signal will result

and the deflection on the output meter will change. This scheme was adapted for use in the meas- urement of standards since it is effectively insensitive to gain variations. This type of radiometer, figure lb, alternately samples power from a known and an unknown noise source by switching

repetitively from one to the other. A null-balance is achieved when the power output from both sources is identical.

The NBS radiometer is modified to compare the noise power from an unknown source and a standard source through a single arm of the radiometer (called the comparison arm) as shown in fig- ure 2. The advantage of this modification is that the paths for the noise power from both the

standard and unknown noise sources are more nearly identical. This advantage means that an insertion loss measurement of two separate paths (c.f. fig. lb) need not be made, and the difference

between the noise sources will be an attenuation difference measurement instead of an absolute

attenuation measurement. Asymmetries in the junction of the arms and time variations in the mismatch of the modulators do not enter into the error analysis.

With a radiometer of this basic design, the signal entering the receiver will have a chopped

appearance unless the power from the reference level source and the source on the comparison arm

have identical power outputs at the receiver input. Figure 3 illustrates schematically the two

different signals seen by the receiver; that is for the conditions of a balance and an unbalance

of noise power between the reference and comparison arms. For convenience of drawing the figure

it is assumed that the resultant signal is a square wave.

2.2 Fundamentals of Error Analysis

When dealing with measurements of random noise, one important consideration that must be

recognized is that there are no passive devices. Devices can be considered passive ONLY when they

are either perfect conductors or they are maintained at a physical temperature of absolute zero

(0 kelvin). The antithesis is that all devices having a physical temperature above absolute zero

are random noise generators; if they have low resistive properties then the noise generation for

any arbitrary fixed temperature will be comparatively small and conversely, devices with high

resistive properties will generate comparatively larger amounts of noise at the same fixed temperature. This being the case, it is necessary to consider maintaining a constant ambient temperature in order to avoid changes in noise generation from a varying temperature affecting separate

devices to different degrees. If noise standards are used that have effective noise temperatures

that are well above ambient temperature then this problem is less severe. With any radiometer the principle of measurement is one of comparing a standard noise source with an unknown noise source. For some applications where only a relative difference needs to be

known, the value of the standard is not needed.

Basically an error analysis is a process of trying to determine: 1) what parameters of a measurement process will constitute an error to the final results; 2) how large will be the con- tribution from the individual components to the total error of each such parameter; 3) how to evaluate each contributing factor; and 4) how to collect the resulting values to arri ve at a meaningful limit to the total error. In the case where a comparison is made between a standard
/-- MODULATOR S-t

STANDARD ATTENUATOR UNKNOWN

SOURCE

RECEIVER

Figure lb.

REFERENCE

LEVEL

SOURCE

MODULATORS-;

/

/

/

ATTENUATOR

RECEIVER

UNKNOWN

SOURCE

STANOARD

Figure 2.

BALANCE UNBALANCE

CARRIER WAVE IS AT MICROWAVE FREQUENCIES

Figure 3.

5
and an unknown, a condition of balance may be arrived at. This will be a special situation where,

at an arbitrary plane of reference, two separate conditions will each produce a balance of the

comparator. A mathematical evaluation of what actually happens at this arbitrary plane for a condition of balance can be made. The equation arrived at will be the’ balance equation for thiit specific comparator. If we make a partial differentiation of this balance equation, we can identify what parameters need to be examined to evaluate their contribution to the total error of the

measurement process. The examination of a parameter must include varying every conceivable variable (e.g., humidity, atmospheric pressure, temperature, power level, frequency, etc.) and any

combination of variables that may affect the parameter itself. Where a mathematical equation is

arrived at, the equation will describe how the errors will be collected, and this result will give

the total systematic error of this measurement process. For the case where random errors are heavily involved, they will need to be collected by a different technique. (The collection of

random sources of error will not be treated here since they provide a very small contribution in

the noise system to be treated.)

2.3 Error Analysis of a NBS Type Switching Radiometer

Using the simplified diagram of the NBS switching radiometer, figure 2, we can isolate our arbitrary reference plane and determine a balance equation. The arbitrary reference plane will be

located immediately before the modulators. For convenience, we will call everything to the left

of this reference plane the receiver. Figure 4 shows what must analyzed.

In figure 4 details of the precision attenuator are elaborated to clarify possible contributions to the total error. is the power entering the input and P 2 the power entering the re- ceiver. In all cases, primed designations indicate that the noise source on the input is the

unknown, and correspondingly the unprimed quantities indicate the standard is attached to the

input. All r's are reflection coefficients and the associated arrow designates which port is

considered. T

n is the noise temperature, and T^ is ambient temperature. S

2 2 is a term of the

scattering matrix and it is proportional to the attenuation of the path [3] from the input plane

(plane 1) to our arbitrary reference plane (plane 2). For the condition where the receiver is in

balance, P 2 = P 2

, where the primes signify that the unknown is attached to the radiometer.

In developing the error analysis it was soon learned that either the attenuator in the com- parison arm had to be matched to the line or to avoid this it had to be calibrated with adequate

isolation on either side. It was chosen to add the isolation to the attenuator as a convenience

to the operator.

A fairly complete analysis of the radiometer is given in the Wells, et al. paper [2]. Using

the general eq (11) from the paper

P

1 = kVf (l-irj 2

) (l-irp)

(3)

Note that P 2 = qPp where

1 S 21 1 2 (i _

1 r

L

1 2

)

|i-s 22r

L

i 2 (i-|r

1

i 2

)

$12$21*~L and T

1 - S n + i-s

z2

V

L (4)

6
7
Following the development in the paper, and using the definition T ne

= T

p

( 1- |

n

| ^ ) (eq (2)) as the

effective noise temperature and the conditions of eqs (3) and (4), the following expression is

obtained.

ne

l(i-r

2

r

L

) ( 1_sn

r

n

) I ld-r’r

L

) d-s n

r

n

)l VT.U-irj 2

)

IS 2l'

2 ^7 (5)

+ + * i re

.

I I Due to having adequate isolation on either side of the attenuator, r

^ - I~ 2 anc* ^il

= Thls means that = 0 and T rg = 0. This modifies eq (5) to

' l 1 _S li r nl

2

l S ?ll

2 Tne

= ^^Tne'

TA (1- |r

n |2)

1 /ji

*11 n

1 |b 21 l

+ T

a(

1-

I r

n

2

)

( 6 )

Differentiating eq (6), an expression is obtained which can be used to determine absolute

I uncertainties. Since ITJ and ITJ are small, the factor (1-ITJ 2

) will be approximately unity.

This gives the approximation for

. IS ?1 I 2, |S ?1 | 2 ST r±— ST + 0.23(T -TJ5A + 1

- —4^— ST, ne - , r ,

ne ne A

7

| S

1

IS 21 ' 21 '

I S pi 1 2

o 0 sir | + 2 T a i-1- irj 2 + | r

' 2 n 'A

* S 21*

2 n n' Tr7

+ M . (7)

ST is the error in T „ or the error obtained in comparing an unknown noise source with a standne ne i ST is the error in the standard noise source,

ne

T is the value of the un- ne ard noise source. known noise source obtained as a result of the measurements and the calculations made using

eq (6). T^ is the ambient temperature of the system and particularly the precision attenuator

21

*21

_ 2 qA/ 10

^ w^ ere ^ j s t h e attenuation difference between the standard and unknown noise

sources as measured on the precision attenuator, so SA is the error in using the precision attenuator. ST^ is the error to which the absolute value of the ambient temperature is known.

6|r

n' -|P | is the error incorporated in measuring the reflection coefficient magnitudes using a tuned

reflectometer [7]. M is the error in determining the mismatch, and its maximum is given by

8
M ( 8 ) =

(i+l s r i ) 2 11 n

|_ (!- 1 s iir

n

I ) 2

- 1 VW'I

All the terms in this equation have been discussed with the exception of S llS 11

is the same as

T

1 since the isolator in front on the attenuator provides at least 30 dB isolation.

These equations are applied successively from the first measurements in which the NBS Refer- ence Noise Standard is used to measure the NBS Working Standards; and eventually to the measurements where the NBS Working Standard is used to measure a customer's Laboratory Noise Standard.

To facilitate understanding this process and to realize the degradation of the measurement ac- curacy, an example is presented later using the NBS Reference Standards and radiometers to determine an Error Flow Diagram.

3. THE NBS SWITCHING RADIOMETERS

3.1 The WR90 and WR62 Radiometers

The WR90 and WR62 radiometers are basically the same. As a consequence this description will

be based solely on the WR90 radiometer. The description will be used as a basis for determining

the accuracy which NBS Working Standards will have.

3.1.1 The WR90 Comparison Radiometer

Figure 5 shows a detailed block diagram of the WR90 radiometer. The comparison arm of the

radiometer consists of a reflectometer [7] in series with a calibrated precision attenuator. The

waveguide switch after the precision attenuation is part of the reflectometer; it connects the

detector and VSWR meter into the circuit for the reflected signal of the reflectometer. Attaching

the reflectometer to the radiometer allows the direct measurement of any noise source attached to the input. When the reflectometer is tuned, the radiometer input port is automatically tuned to approximate a nonreflecting load. As far as the error analysis is concerned the presence of the

reflectometer adds nothing to figure 4. The calibrated precision attenuator section is removable

for calibration by the subcarrier technique to an accuracy of ± 0.005 dB; the attached isolators

eliminate the need for matching the attenuator to the line [2]. The attenuator itself is a rotary-vane attenuator that is modified by a gearbox that is permanently mounted to it. The com- bination provides a fine control of the rotor of the attenuator with no detectable backlash so that a 0.001 degree movement of the rotor is resolvable. The angular movement of the rotor is

converted to attenuation values [8] quite easily, thereby providing resolution capability at low

attenuation values to 0.0001 dB, and at high attenuation values to 0.001 dB.

The reference arm is composed of two attenuators and the reference level noise source. One

attenuator is used to balance the power in the reference arm with that entering the receiver from

the comparison arm. The other attenuator is used for the purpose of determining the sensitivity

of the receiver by introducing small known changes to the receiver input.

The switching method employed in the WR90 and WR62 radiometers is a mechanically driven pair

of modified rotary-vane attenuators. The modification of the attenuators amounts to substituting

9
X

10

H>IVh

'OriHV'INO.l
the bearings for a variety more suited to continuous rotation and to replacing the manufacturer's

drive mechanism by toothed pulleys and timing belts [1]. The orientation of the vanes of the

attenuators is such that they maintain a 90° angular difference; the resulting effect is a switching system where the receiver sees only one arm of the radiometer at a time. The combining

junction for the two arms is a directional coupler, and the output of the modulators is a 30 Hz modulation of the rf noise signal.

The signal enters the mixer-preamp via a low pass filter, which limits the bandpass, cutting

off higher frequencies of noise power. This prevents undesirable higher harmonics being generated

when the signal is beat with the signal from the local oscillator. Both the response and the

image components of the intermediate frequency are used. The waveform of the IF is the same as shown in figure 3, except that now the carrier frequency is at 30 MHz. Since the bandwidth of the

IF amplifier system is approximately 8 MHz, the assumption is made that the noise spectrum of the

microwave noise sources being used is flat over a minimum of 68 MHz, figure 6. This assumption is

usually valid. The IF signal is amplified and detected, thereby sending a 30 Hz signal to the

synchronous amplifier. The 30 Hz signal is first amplified and then detected synchronously *t the

rotary-vane switching frequency. The resulting signal looks like a rectified 30 Hz signal which

will assume a positive or negative attitude, when the radiometer is unbalanced, and appear to have

no signal, when the radiometer is balanced. This type of detection scheme inherently has a very narrow bandwidth. The d-c component is obtained by filtering this signal and displaying the re- sult on a recorder.

Some refinements of the system that have resulted in the better operation are: 1) the local

oscillator frequency is stabilized by a servo-control system using phase detection; 2) the filament power supply for the mixer-preamp and IF amplifier is a regulated dc voltage supply; 3) a waveguide switch is included in the local oscillator arm so that local oscillator power can be

turned off to prevent overdriving the system when changing from the standard noise source to an unknown noise source, and to use the local oscillator power for the reflectometer, eliminating the

need for a separate power source; 4) the mixer-preamp is isolated from the L.0. and the rest of

the radiometer's waveguide by a dc block (made from mica); and 5) the whole system is rigidly

mounted on a secure base, (fig. 7).

The measurement technique is as follows. An unknown noise source is connected to the radiometer with the precision attenuator set at a calibrated setting. The balance attenuator is adjusted until a null output is observed on the scope and recorder. A sensitivity check may be made

by inducing a known change on the sensitivity-balance attenuator with which the recorder is calibrated. After a null is reached the local oscillator power is turned off and the unknown noise

source is exchanged for a standard noise source. The local oscillator is turned on and the precision attenuator is readjusted until the null is again achieved. The amount of attenuation that

was introduced or removed by adjusting the precision attenuator is directly related to the difference in the output noise power of the standard and the unknown. The resulting value of the

effective noise temperature of the noise source to be measured may be calculated by applying thp

data to:

I (T

ne V10A/10

+ T, (9)

11
f 0

= L.O. FREQUENCY

n

H h- 8 MHz

[• 30 MHz

8 MHz

•30 MHz *|

•68 MHz

Figure 6.

Figure 7.

12
where is the measure of the ambient temperature of the precision attenuator, A is the algebraic

difference in attenuation as read on the precision attenuator, and T ne

is the effective noise

temperature of the standard. It should be clear that this is a reduction of eq (6) when the as- sumption is made that |T | and ITJ are quite small, and I

21 l 2 /|

21 l 2 =

From this example we can see that the most critical requirement is in having a high quality

precision attenuator in the comparison arm. This attenuator must have good resolution, high precision, and an accurate calibration. Further, it will be noted that the only requirement on the

"reference level" noise source is that it must be stable over the length of time necessary to observe the difference in output of both the standard and the unknown. The difference between

these two noise sources is determined at least five times, and an average of the results is com- puted and reported to the customer.

3.1.2 Applying the Error Analysis to the WR90 Radiometer

In examining the radiometer, care is taken to assure that no d-c bias exists in the RF detection scheme, no harmonics are generated in the mixer, the L.O. frequency is adequately stable

and has minimal noise input, and the synchronous detector is synchronized to the switch.

The ambient temperature is evaluated to determine what value to assign to 6T^. In the NBS

lab the ambient temperature varies throughout the room so the temperature is monitored in the

vicinity of the precision attenuator. An evaluation was made to determine the difference between

the temperature at the monitor and the temperature of the attenuating vane in the precision attenuator. It was found that ST^ is less than 2°C.

An analysis was performed on the built-in ref 1 ectometer to ascertain the limits of error in

6 |r

n

l IP

. . A maximum error of 5 percent was found [7].

n 6A contains all errors in determining A. These include the limits of error assigned to the

attenuator during its calibration process (which include the discrepancy in attenuation values

read in the 68 MHz bandwidth of concern), the error in distinguishing a null, and the error due to the operator judgement. The error is less than 0.005 dB. The latter errors are determined by

experimentation.

The mismatch error is arrived at by using eq (8) with the measured values obtained. If the

receiver uses both the image and response of the IF, then all F and S values must be evaluated at the separate IF frequencies. For example, the WR90 Radiometer uses a 30 MHz IF; this means the r and S values must be measured at f + 30 MH, and f - 30 MH . If the values measured are large,

o z o z

3 then further measurements are necessary to see how r and S vary in the bandwidth of concern about

both f ± 30 MHz. If the values measured are small, they can be averaged to get a value for use

in the equations.

The value of 6T

ng

is the limit of error to which the standard being used to perform the calibration is known. For an example , let us assume the NBS Reference Noise Standard is known to approximately ±4.0 kelvins at 9.6 GHz; this standard is used to measure and evaluate the NBS

Working Standards. For an NBS Working Standard that has an effective noise temperature of

11,000 K and a reflection coefficient magnitude of 0.02 we can apply the equations to see that

during the process of comparison a degradation takes place and the NBS Working Standards are correspondingly known to approximately ± 109.4 kelvins. This is the applicable value used to

13
determine the limits of error of the unknown interlaboratory standard. If we assume the unknown

I interlaboratory standard sent for calibration is stable, has a IFJ < 0.02, and yet has a 100 K different effective noise temperature from the NBS Working Standard then the limit of error will

be ± 136.4 kelvins. This example is worked out in table 1 with typical values for a series of

comparisons at 9.6 GHz. The same information is included in an Error Flow Diagram in figure 8. This diagram illustrates that each time the radiometer is used a degradation of error results.

Note that although the NBS Reference Noise Standard is known to ± 4.0 K, after making a comparison

the effect is that ± 43.9 K or 40.1% of the error of the NBS Working Standard is due to the Refer- ence Noise Standard.

Table 1

A. Using the NBS Reference Noise Standard to calibrate an NBS Working Standard (typical values

for 9.6 GHz).

ST = 4.0 K, ne

Ta = 24. 8°C = 298 K, 6T

a = 2 K,

ir

n

i < 0.07 ir

n

i < 0.02, s n = o.oi

= ID

1 ' 04 = 11,

' S2l'

2

6 1 r

' r

n'

= 0.05

T ne = 11,000 K 8A = 0.005 + 0.005 = 0.01 dB (for Working Standard).

6A is comprised of the limits of error of the attenuator's calibration (±0.005 dB), and the

balance and operator error (when using the Reference Standard these two amount to ± 0.005

dB). Using eq (7)

6T ' ~ 43.9 + 24.6 + 20 + 1.6 + 19.3 = 109.4 K ne -

B. Using an NBS Working Standard to calibrate a customer's Interlaboratory Standard.

ST = 109.4 K, neT a = 298 K 5T

a = 2 K

T

ne = 11,100 K (for Customer's Interlaboratory Standard),

s 11 = 0.01, ir n

l = ir ni < 0.02

!S 2il

2 —I S 1 2 zl

,0.004

sir

= io

u ' uu^ = 1.0093, -jyA- = 0.05, SA = 0.005 + 0.002 = 0.007 dB

n

(balance and operator error in this case is ± 0.002 dB). Using eq (7)

ST ~ 110.4 + 17.4 + 0 + 0 + 8.6 = 136.4 K. ne -

14
ERROR FLOW DIAGRAM

(FOR 9.6 GHz AND VALUES IN TABLE I)

[CUSTOMER'S INTERLABORATORY STANDARD +136.4 K = 1.2% j

Figure 8.

15
Figure 9 gives an idea of the accuracy to which two of the four NBS Working Standards are

known. The particular two NBS Working Standards have been measured and evaluated at five frequencies and their limits of systematic error have been determined through the process related.

It is easily seen that the accuracy improves as the frequency increases across the band. The WR62

NBS Working Standards have comparable curves with comparable values. Since the customer's standard is calibrated against the NBS Working Standard it too will display a similar increase in ac- curacy across the band although the differences from one end of the band to the other will depend

upon the characteristics of the item being measured.

4. WR90 AND WR62 MEASUREMENT SERVICES

4.1 Existing Waveguide Measurement Services

At present two switching systems exist for the measurement of high- temperature sources and

they are in the WR90, and WR62 waveguide sizes. The word system is used to include the appropriate NBS Reference Standard, NBS Working Standards, and the radiometric comparator.

Although the systems are constructed in WR90 and WR62 waveguide sizes, the items that can be

calibrated by them can be in any of the waveguide sizes whose frequency range overlaps that of one

of these systems ... namely, WR112, WR75, and WR51. To measure noise sources in these other waveguide sizes, a waveguide to waveguide transition is required. The transition is analyzed to determine the loss incurred in attaching it to a customer's interlaboratory standard and to determine

the additional error it will cause to the calibration of the noise source. Table 2 is extracted from the NBS Special Publication No. 250 [9] and describes what calibration services of noise sources using the WR90 and WR62 systems are available. Extension of these

services to cover frequencies above the normal upper limits of each of the systems is not possible

due to problems of multimoding. Extension in a downward direction can be handled as long as the

requested frequency is at least 0.15 GHz above the cutoff frequency of the waveguide.

Table 2 identifies the NBS Fee Schedule number, and the output noise power range in both

effective noise temperature and excess noise ratio that can be handled by the calibration service.

This range of noise power output can be increased easily in an upward direction to cover solidstate type noise sources if the need arises. To extend the range in a downward direction is not

as easily accomplished. An indication of accuracy is given in terms of both effective noise temperature and excess noise ratio. The actual accuracy reported is based upon the measurements.

To establish a frequency for regular use in the calibration service takes 3 to 4 months for

the necessary preliminaries, primarily reactivating the reference standard and evaluating it at the specified frequency, then measuring and evaluating the working standards (which includes evaluation of the radiometer and calibration of the precision attenuator and cavity wavemeter). Due

to this lengthy effort a selection of frequencies have been established in each waveguide size

(table 3) to try and cover the frequency band as well as possible and eliminate the need for con- tinually adding frequencies. On occasion the need for a specific frequency will exist either to get the greatest accuracy or to do work at a classified frequency, and NBS will accommodate these

needs under fee schedule 4.6.1 as it is able to do so.

16
FREQUENCY IN GHz

Figure 9.

17
Table 2

EFFECTIVE NOISE TEMPERATURE MEASUREMENTS OF WAVEGUIDE NOISE

SOURCES USING THE WR90 AND WR62 SYSTEMS

Service available: Effective noise temperature.

NBS Fee Schedule Waveguide Size

Effective Noise

Temperature Range

4.6D WR 62(12.4-18.0 GHz)

WR 75(10.0-15.0 GHz)

WR 90(8.20-12.4 GHz)

WR112(8. 20-10.0 GHz)

700-300,000 K 700-300,000 K 700-300,000 K 700-300,000 K

Excess Noise

Ratio Range

1.5 to 30 dB

1.5 to 30 dB

1.5 to 30 dB

1. 5 to 30 dB

Special Calibration

Source Impedance Limits of Error

Reflection Effective Excess

Waveguide Size

Waveguide

Flange VSWR

Coefficient

Magnitude

Noise

Temperature

Noi se Ratio

WR 62(12.4-18.0 GHz) UG-419/U < 1.2 < 0.09 ± 150-200 K ± 0.06-0.08 dB

WR 75(10.0-15.0 GHz) Cover type < 1.2 < 0.09 ± 175-225 K ± 0.07-0.09 dB

WR 90(8.20-12.4 GHz) UG-39/U < 1.2 < 0.09 ± 150-200 K ± 0.06-0.08 dB

WR112(9. 20-10.0 GHz) UG-51/U < 1.2 < 0.09 ± 175-225 K ± 0.07-0.09 dB

Limits of error for waveguide noise

tures of approximately 11,000 K. sources are estimatec1 for effective inoise temperaTable 3

FREQUENCIES AVAILABLE FOR CALIBRATION IN THE WR90 & WR62 BANDS

WR90 WR62

in GHz in GHz

8.2 12.4

9.0 13.5

9.5 14.0

9.8 15.0

10.0 16.0

10.2 16.5

10.5 17.0

11.2 18.0

11.8

12.4
REFERENCES

[1] Estin, A. J. , C. L. Trembath, J. S. Wells, and W. C. Daywitt, "Absolute Measurement of Temperatures of Microwave Noise Sources," IRE Trans, on Instr. , Vol. 1-9, pp. 209-213, Sept.

1960.

[2] Wells, J. S. , W. D. Daywitt, and C. K. S. Miller, "Measurement of Effective Temperature of

Microwave Noise Sources," IEEE Trans, on Instr. and Meas. , Vol. IM-13, pp. 17-28, March 1964.

[3] Kerns, D. M. and R. W. Beatty, "Basic Theory of Waveguide Junctions and Introductory Micro- wave Network Analysis," New York, Pergamon Press, 1967.

[4] "IRE Standards on Electron Tubes: Definition of Terms 1962 (62 IRE 7.52)," Proc. IEEE, Vol.

51, pp. 434-435, March 1963.

[5] a. Mukaihata, Tadao, "A Survey of Techniques in Microwave Noise Measurement, ISA Trans.,

Vol. 3, pp. 342-353, Oct. 1964.

b. Haneman, F. G. and A. J. Hendler, "Automatic Indication of Receiver Noise Figure, Topics

in Noise," Airborne Instruments Laboratory, Aug. 1960.

c. "Noise Figure Primer," Hewlett-Packard Co., Application Note No. 57, June 1962.

[6] Dicke, R. H. , "The Measurement of Thermal Radiation at Microwave Frequencies," Rev. Sci.

Instr., Vol. 17, pp. 268-275, July 1946.

[7] Anson, W. J. , "A Guide to the Use of Modified Reflectometer Techniques of VSWR Measurement,"

Journal of Research of NBS, Vol. 65C, No. 4, pp. 217-223, Oct. -Dec. 1961.

[8] Larson, W. , "Table of Attenuation as a Function of Vane Angle for Rotary-Vane Attenuators,"

NBS Tech. Note 229, Jan. 1965.

[9] a. "Noise Sources in Four Waveguide Sizes," NBS Tech. News Bull., Vol. 53, No. 1, p. 16, Jan. 1969.

b. "Effective Noise Temperature Measurements of Waveguide Noise Sources," NBS Spec. Publ.

250 (Calibration and Test Services of the National Bureau of Standards), p. 757A revised

Nov. 1968.

19
NBS-114A (rev. 2-80

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET (See instructions)

l. PUBLICATION OR

REPORT NO. NBSIR 84-3004

2. Performing Organ. Report No. 3. Publ i cation Date

May 1984

4. TITLE AND SUBTITLE

THE NBS SWITCHING RADIOMETERS

5. AUTHOR(S)

C.K.S. Miller and W.C. Daywitt

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered WASHINGTON, D.C. 20234 Final

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10.

SUPPLEMENTARY NOTES

J Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. I f document includes a significant

bibliography or literature survey, mention it here) An error analysis for the Dicke radiometers used by the National

Bureau of Standards (NBS) in their IJR90 waveguide noise calibration

services for sources with noise temperatures above 1000 kelvin is

discussed. A list of measurement frequencies currently available in the

WR90 and WR62 bands is presented.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolons)

error analysis; microwave; noise; radiometer

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